Anodes have been used commercially for many years in electrolytic processes for the preparation of various chemicals such as chlorine, bromine and hydrogen peroxide, for the electrodeposition of metals such as chromium, copper and zinc, as well as for high speed electroplating such as electrogalvanizing.
The conventional electrolytic anode consists of a substrate made of a valve metal, such as titanium, niobium, tantalum or zirconium or an alloy of these metals, and an electrocatalytic coating of a precision metal(s) or precious metal oxide(s), where the precious metal is usually a platinum group metal, such as iridium, platinum, rhodium or ruthenium. The precious metal or metal oxide coating is often mixed with the oxides of the valve metals. Typically, the valve metal substrate is also subjected to a surface treatment such as chemical etching, mechanical gritblasting and/or the application of a wash coat, prior to the electrocatalytic coating. The electrocatalytic coating is also typically applied by either electrodeposition or thermal deposition methods. Also, with the development of new high speed electrogalvanizing processes, where extremely low pH, high current densities and elevated temperatures are employed, a barrier layer has been introduced to protect the valve metal substrate from its passivation.
For example, U.S. Pat. No. 4,203,810 to Warne discloses an anode for use in an electrolytic process comprising a substrate of titanium, tantalum, or niobium over which a barrier layer containing platinum or platinum-iridium alloy is formed by painting a chemical compound containing platinum and iridium over the substrate, the painted substrate subsequently being heat treated. A layer of a precious metal is applied over the anode by an electroplating process.
Similarly, U.S. Pat. No. 4,331,528 to Beer discloses an anode having a film forming substrate of titanium, tantalum, zirconium, etc. over which a thin barrier layer is formed. The barrier constitutes a surface oxide film grown up from substrate that also incorporates rhodium or iridium metal or their compounds in an amount of less than 1 g/m.sup.2 (as metal). The anode is then thermally coated with an electrocatalytic coating comprising at least one platinum-group metal or metal oxide possibly mixed with other metal oxides, in an amount of at least about 2 g/m.sup.2.
Additionally, U.S. Pat. No. 4,528,084 to Beer discloses an anode having a barrier layer formed over a substrate from a solution containing a thermo-decomposable compound of a platinum-group metal and also a halide which attacks the substrate which purportedly results in increased performance.
U.S. Pat. No. 4,913,973 to Geusic discloses an anode comprised of a valve metal substrate over which a barrier layer consisting of at least 150 .mu.inches of electroplated platinum is formed. The barrier layer is subsequently heated at high temperatures to reduce the porosity of the barrier layer. A second thermally deposited coating of iridium oxide is subsequently deposited over the barrier layer.
U.S. Pat. No. 5,672,394 to Hardee describes an anode with a surface roughness of at least 250 microinches (6 microns) and an average surface peaks per inch of at least 40 that has a ceramic barrier layer followed by a thermally deposited electrocatalytic coating composed of a mixture of iridium and tantalum oxides.